Solderless SMA screw mounting RF issues

[yefj]

Hello,I have a PNA which works till 120GHz I use a 1mm to 1.85mm adapter to measure till 67GHz 1.85 SMA.

I need to measure a differential lines.
At first i got low causality about 50% so i played with the connector screwed unscrewed tried wipe of some spots i saw on the ground pad.
Then i saw that the causality got to almost zero and i cant improve it back to the values before.
I tried to changes cables to shorter ones.
I dont know if my 1mm to 1.85mm screwed proprly to the PNA.
I dont know how to place propectry the SMA and how much presure to put on the screws so it will work properly.
Any help?
photos attached.
Thanks.

 

[berkeman]

Why are there screws to connect the SMA connector to the PCB? It should just be soldered, no? Have you contacted the connector manufacturer to ask these questions? What is the part number of the connector and please give a link to it at the manufacturer's website. Thanks.

 

[sophiecentaur]

At first i got low causality about 50%

I have recently noticed the term "causality" used in items about component with multiple transmission lines. In my past experience, the term 'Group Delay' seems to have been used as a measure of the behaviour of pulse signals in a circuit. For instance,video circuit performance used graphs of amplitude and group delay response to specify requirements. Is this change due to the frequencies and bandwidths involved or is it just a matter of fashion?

 

[Baluncore]

I have recently noticed the term "causality" used in items about component with multiple transmission lines. In my past experience, the term 'Group Delay' seems to have been used as a measure of the behaviour of pulse signals in a circuit. For instance,video circuit performance used graphs of amplitude and group delay response to specify requirements. Is this change due to the frequencies and bandwidths involved or is it just a matter of fashion?

"Causality" is a product of the way that the TDR can be computed from the S parameters.

The resolution of TDR was limited by the rise time of the incident step function, and the internal multiple reflections within the network.

Given a frequency sweep of a cable, done with a network analyser, it is possible to compute from S11, the impedance of the network, at different ranges, or times, into the network. The result is limited by the frequency band swept by the analyser, and the resolution of the frequency steps taken by that analyser, within that band.

That computation starts from the known impedance of the adapter cable between the analyser and the network under test. That adapter cable can be computationally removed. When the TDR is computed, an estimate of "causality" shows how reliable the computed TDR of the network can be.
https://www.signalintegrityjournal....ling-and-bandwidth-shape-time-domain-fidelity

 

[sophiecentaur]

"Causality" is a product of the way that the TDR can be computed from the S parameters.

OK. Is this different from the results of under sampling that we all know and love? I imagine that this is an issue is that there is a limited length of time available for a frequency sweep of the S parameters and that results in under sampling in the frequency domain, which results, after the Fourier transform, in unexpected errors in the time domain which are not as straightforward as the familiar and identifiable 'alias components' that give out of band components and distortion in-band.

I imagine that the problem is there because of practical problems generating and sampling a direct time domain response graph. Those problems are mirrored in the limited bandwidth of the 'sample' frequency steps / samples. So the bottom line will be the resulting SNR and Error Rate in the data.

I haven't understood why, because causality must always apply in reality, an imperfect simulation cannot predict the overall delay of the first sample through the system. If multiple signal paths are involved then the delays must be equalised - to an 'acceptable' degree. Perhaps this 'acceptable' criterion will depend on the actual shape of data pulses and even the pattern of the data. I looked at the reference and it seems that they use the term 'causality' to calculate the errors - hence their comparison between the causal and non-causal treatment.

All this is reminiscent of analogue colour TV circuit design where the image on the screen is basically a time varying electron beam current which (for a stationary picture) appears as distance variation of phosphor brightness which poses demands in both frequency response and time response (edges and patterns). Simulating that in the old days was demanding and test signals had to be pretty ingeniously designed.

 

[yefj]

Hello Balun core ,i cant find nowhere the torque needed to connect the cable to the connector and what toque to use .
can you give some reccomendations on the subject?
Thanks.

 

[sophiecentaur]

I dont know how to place propectry the SMA and how much presure to put on the screws so it will work properly.

It's possible that the solderless type connection is just not up to the spec that's really needed. Pictures I see of SMA connectors on circuit boards all seem to used soldered connectors so that may be a 'message' about screwed types.

There are many versions of RF connectors which are cheaper than other versions and they have inferior specs, even though they have the same mechanical dimensions and will fit high quality cables.

 

[Baluncore]

What is the part number of the connector and please give a link to it at the manufacturer's website.

Hello Balun core ,i cant find nowhere the torque needed to connect the cable to the connector and what toque to use .
can you give some reccomendations on the subject?

I would need to know the manufacturer and the part number of the connector.
It appears the connector is physically attached to the PCB with two screws.
How are the electrical connections made between the connector and the PCB?

 

[sophiecentaur]

How are the electrical connections made between the connector and the PCB?

This is a crucial point. SMA connectors have a long history and have a wide range of applicable frequencies.
SMA is very convenient as a connector because it's small and handy to fit into situations. SMA has been produced for many years and started with a maximum usable frequency of 12GHz but now work t 26GHz What actual frequency is the OP using and is the measured problem relevant at his actual operating frequency?

@yefj Your Analyser can give you S parameters for frequencies which may be outside of the range you are using. If the solderless connection continues to give trouble then consider a soldered connector and also pay attention to the circuit board tracks to ensure a good continuous impedance from cable to the board. Good Engineering requires identify the relevant problems and solve them, one at a time, in order of importance. This often needs experience.

 

[yefj]

Its end launch 1.85 connector.
I have wera 851/9C 1 PHO which is Ncm, 8in-lb is 11Ncm.If i put 11 its too hard on the connector I feel.
Is there a way In the VNA to see if the connector is propely connected?

 

[Baluncore]

Is there a way In the VNA to see if the connector is propely connected?

Use TDR mode to look at the internal impedance of connectors.

 

[Baluncore]

I dont know if my 1mm to 1.85mm screwed proprly to the PNA.
I dont know how to place propectry the SMA and how much presure to put on the screws so it will work properly.

Your PNA instrument has 1.0mm connectors, good from DC to 120GHz.
You adapt the 1.0mm to 1.85mm which is then good only to 70GHz.
You do NOT use SMA (≤18GHz), which is quite different from the 1.85mm system. The 1.85mm and SMA look very similar, but have different dimensions and threads, so they cannot be reliably mated.

If you use the wrong connector you may deform the mate, so it will not contact correctly again without correction or replacement. As an example: If you use a 50Ω BNC male to connect to a 75Ω BNC socket, it will seem to work with only a 50:75 impedance mismatch, but the socket may be physically spread, sufficiently to make unreliable contact with a 75Ω BNC male in the future.

The connector image you show in post #1, looks like a "vertical launch" connector, that has an angle of 90° between the trace and the connector axis. It is not an "end launch" connector, one that mounts on the edge of the PCB keeping the connector axis in line with the trace on the PCB. The 90° bend may restrict the bandwidth. Or maybe that is the SMA you refer to, that does not mate with a 1.85mm connector?

 

[sophiecentaur]

Use TDR mode to look at the internal impedance of connectors.

You need to intermpret what that shows you. Massive 'bumps' in the trace may have no affect at all then they are short and 'balanced' by other bumps; in fact you'd need to look at the effect of those bumps in isolation when passing the sort of pulses you intend to be using.

 

[sophiecentaur]

Is there a way In the VNA to see if the connector is propely connected?

Cleaning the contact face or examining for distortion could reveal any obvious problem. Also, changing the connector for a new one or, as I already commented, try a soldered version.

A thorough internet search (AI) for work on the problem of connection to a circuit board could be good value. You need to find someone who's actually had similar problems and solved them.